Some set stitch length to 0 and others dont. Some lower feed dogs and others don't.
I prefer to lower the feed dogs then they don't scratch the back side of the quilt. I don't understand why some are so adamant about lowerig and not lowering.
The reason some give for setting stitch length to 0 is that disengages the feed dog mechanism and less wear on that part of the machine. Quilters find different ways of doing fmq and work to find the best method for themselves.
If you don't lower the feed dogs and leave the stitch length as set on the machine then the dogs will be pulling the fabric one way and you are pulling or pushing the quilt the other. I believe leaving them up will cause the bottom to drag and not be easy to slide under the pressure foot. So for me the safe way is to lower the feed dogs and leave the stitch length where it is set on the machine. And......either way your stitch length is determined by the speed of your machine and the speed in which you move the quilt. I try and recommend working to a stitch lenth about what the normal sewing would be or just a bit longer. I try for an even length of about what a 3 setting on your mcahine would be. I believe you need the slightly longer stitch length to accommodate the additional thicknesses the batting adds to the quilt. Tiny stitches make the quilting stiffer and hold the layers more tightly together and can cause some dimensional appearance not suitable to the design. I also find the longer stitches gives me more freedom in motion of moving the quilting around. I also find that students are more satisfied with their work and learn faster when the stitches are longer. They ask why their motifs are not smooth and even and in most cases it is because they have very tiny stitch length and when they strive for longer stitches their work improves.